Integrated circuits such as, for example, optoelectronic sensors, are typically manufactured as packages in multiple quantities formed from “chips” fabricated on printed circuit boards. These packages are constructed by forming integrated circuit dies on a FR4 printed circuit board or Bismaleimide-Thriazine (BT) substrates. The die is wire bonded with, for example, gold wire and then over molded with epoxy to protect the die and the wire bonds. The substrate is then cut into small pieces using a sawing process to form individual integrated circuit packages.
The over mold process utilizes a transfer molded epoxy material. The epoxy material and the substrate on which the integrated circuit dies are formed, have different thermal coefficient of expansions. The combination of the epoxy material and the substrate and the tooling design methods utilized, create large stresses in the individual integrated circuit packages. Stresses in the individual packages are caused by the difference in thermal coefficient of expansion between the mold epoxy and the substrate. The stresses cause the substrate to curl after over molding. As a result of the curling, fixturing is necessary to flatten the substrate prior to the sawing operation.
A need has thus arisen for a substrate and fabrication process which provides additional strength for the substrate to prevent undesired curling of the substrate prior to a sawing operation.